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FreeBSD-SA-13:08.nfsserver                                  Security Advisory
                                                          The FreeBSD Project

Topic:          Incorrect privilege validation in the NFS server

Category:       core
Module:         nfsserver
Announced:      2013-07-26
Credits:        Rick Macklem, Christopher Key, Tim Zingelman
Affects:        FreeBSD 8.3, FreeBSD 9.0 and FreeBSD 9.1
Corrected:      2012-12-28 14:06:49 UTC (stable/9, 9.2-BETA2)
                2013-07-26 22:40:23 UTC (releng/9.1, 9.1-RELEASE-p5)
                2013-01-06 01:11:45 UTC (stable/8, 8.3-STABLE)
                2013-07-26 22:40:29 UTC (releng/8.3, 8.3-RELEASE-p9)
CVE Name:       CVE-2013-4851

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.

I.   Background

The Network File System (NFS) allows a host to export some or all of its
file systems so that other hosts can access them over the network and mount
them as if they were on local disks.  FreeBSD includes both server and client
implementations of NFS.

II.  Problem Description

The kernel incorrectly uses client supplied credentials instead of the one
configured in exports(5) when filling out the anonymous credential for a
NFS export, when -network or -host restrictions are used at the same time.

III. Impact

The remote client may supply privileged credentials (e.g. the root user)
when accessing a file under the NFS share, which will bypass the normal
access checks.

IV.  Workaround

Systems that do not provide the NFS service are not vulnerable.  Systems that
do provide the NFS service are only vulnerable when -mapall or -maproot is
used in combination with network and/or host restrictions.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch http://security.FreeBSD.org/patches/SA-13:08/nfsserver.patch
# fetch http://security.FreeBSD.org/patches/SA-13:08/nfsserver.patch.asc
# gpg --verify nfsserver.patch.asc

b) Apply the patch.

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:http://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/8/                                                         r245086
releng/8.3/                                                       r253694
stable/9/                                                         r244772
releng/9.1/                                                       r253693
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing XXXXXX with the revision number, on a
machine with Subversion installed:

# svn diff -cXXXXXX --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing XXXXXX with the revision number:

<URL:http://svnweb.freebsd.org/base?view=revision&revision=XXXXXX>

VII. References

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-4851>

The latest revision of this advisory is available at
http://security.FreeBSD.org/advisories/FreeBSD-SA-13:08.nfsserver.asc
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